One of the most common diabetes drugs is a nutritional vampire, sucking a key vitamin right out of the millions unlucky enough to be taking it.

Researchers say metformin can rob the body of vitamin B12 — a critical nutrient that helps balance your mood and memory. If you’ve ever wondered why diabetics are so gloomy, there’s your answer.

Until now, most doctors have assumed that those famous diabetic mood swings were caused by the disease… not the drugs.

Whoops — wrong again! The study in the British Medical Journal shows us what’s really going on here.

Researchers randomly assigned 390 diabetics to either metformin or a placebo for four years. Those lucky enough to get the placebo had no change in their B12 levels… but the metformin crowd got their own personal raincloud: A 19 percent drop in B12 levels.

What’s more, the researchers say the B12 robbery got worse over time. The study cut off after four years — so who knows how badly deficient you might be if, like so many other diabetics, you’ve been taking this med for a decade or more.

If you’re a diabetic, the best thing to do is quit sugar, get your life back under control and get off these meds as soon as you can. And while you work on that, make sure you’re getting enough B12.

That means more fatty steaks, and ZERO snack cakes.

And if you’re not working on the lifestyle changes that’ll get you off your diabetes meds, don’t even worry about your B12 levels. People who depend on these drugs inevitably die of a diabetes-related condition sooner or later, and a few extra vitamins won’t make much of a difference.